The detection of the sky in an image enables easier understanding of the image content with a view to performing later digital processing.
Thanks to the development of data processing, especially digital processing tools, methods have been developed to index each pixel of an image, according to a color criterion, in order to identify certain categories of objects in the image, such as the skin, grass or sky. The first techniques used are generally statistical methods based on pixel by pixel color analysis. In this spirit, a more elaborated technique has been developed, using color spectral analysis, based on histograms of shades from which the brightness and saturation components have been removed. However, this sophistication only provides a limited improvement, due to the purely statistical approach and use of a single color criterion.
Another approach has developed in parallel, which includes image structural analysis, for which segmentation into several zones is performed based on color criteria. Compared with purely statistical techniques, this approach provides an improvement, due to the structural analysis of the image. Nevertheless, it remains limited because it is only based on color criteria. The development towards more sophisticated techniques, based on several criteria, such as color and texture, have enabled significant improvements. However, the processing quality usually remains linked to the presence, in the processed image, of an object used as color reference. The semantic classes into which it is planned to classify objects are difficult to reduce to a set of low level characteristics. Thus this approach remains inaccurate despite the multiplication of criteria. The most recent techniques integrate the use of physical models, which enables more accurate distinction among the various objects of an image, and especially, in the identification of sky zones.
Patent Application EP 1,107,179 describes a method to detect regions of sky in an image, based on a prior classification of the image pixels according to a color criterion, its segmentation and sky identification by physical criteria.
The method described enables the sky to be differentiated from objects having the same color and texture as it.
However, this method only enables the detection of regions having a uniform blue color, which excludes the detection of a cloudy sky.
It is an object of the invention to provide a process that enables the identification of a blue, cloudy or overcast sky in a color image.